Device for measuring displacement

ABSTRACT

Herein disclosed is a device for measuring a displacement of a movable member relative to a fixed member by putting the electromagnetic induction into practical use. The device comprises a first coil having first and second coil sections which are coaxially aligned along a common axis, a second coil arranged to establish a magnetic coupling with the first coil and extending along the common axis, and a short circuit ring arranged to establish a magnetic coupling with the first and second coils and movable relative to the same. The winding density of each of the first and second coil sections is gradually decreased from the outside end to the inside end thereof, and the winding density of the second coil is substantially even throughout the length of the same.

This application is a division of application Ser. No. 845,537, filedMar. 28, 1986 now issued as U.S. Pat. No. 4,723,446.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a device for measuring adisplacement of a movable member relative to a fixed member, and moreparticularly to a device for measuring the displacement by using theelectromagnetic induction phenomenon. More specifically, the presentinvention is concerned with a device for detecting a liquid level in acontainer by putting the electromagnetic induction phenomenon intopractical use.

2. Description of the Prior Art

As one of the conventional liquid level detecting devices, there hasbeen proposed such a device as disclosed in FIG. 1 of the attacheddrawings. The device "A" disclosed is of a so-called "variableresistance type" arranged in a container in which a liquid to bemeasured is contained.

The device "A" comprises a zig-zag resistor pattern "B" and an elongateconductive pattern "C" which are arranged in parallel and printed on arectangular insulating base plate "D". The base plate "D" is fixed inthe container to extend vertically. A float E is disposed about the baseplate "D" so as to be vertically movable relative to the fixed baseplate in accordance with a rise and fall of the level of the liquid inthe container. Two conductive sliders "F₁ " and "F₂ " are carried by thefloat "E" with their leading ends slidably contacting with the twopatterns "B" and "C", respectively. The two patterns "B" and "C" and thesliders "F₁ " and "F₂ " thus constitute a variable resistor theresistance of which changes in accordance with the vertical movement ofthe float "E", that is, in accordance with the fluctuation of the liquidlevel in the container. The zig-zag pattern "B" (viz., the measuringresistor) is connected to an electric power source "G" through a fixedresistor "H". The voltage fluctuation thus appearing between themeasuring resistor "B" and the fixed resistor "H" due to the fluctuationof the liquid level is detected by a voltage detecting circuit "I" andthe voltage fluctuation thus detected is treated by an indicatingcircuit "J" to indicate the amount of the liquid in the container.

However, the above-mentioned device "A" has suffered from the drawbackthat due to the mechanical contact between each slider "F₁ " or "F₂ "and the printed pattern "B" or "C", long use of the device inducesremarkable wear of the measuring resistor "B" changing the originalresistance value of the same. This causes erroneous measuring of liquidlevel. In the severest case, the measuring pattern "B" is broken becauseof the wear.

Furthermore, due to a friction inevitably produced between each slider"F₁ " or "F₂ " and the printed pattern "B" or "C", the upward ordownward movement of the float "E" is not smoothly carried out therebycausing erroneous indication of liquid level. This undesirablephenomenon becomes severer when foreign matter gets in between thesliders and the printed patterns accidentally.

SUMMARY OF THE INVENTION

It is therefore an essential object of the present invention to providea measure for solving the above-mentioned drawbacks.

It is another object of the present invention to provide a device formeasuring a displacement of a movable member relative to a fixed memberby practically using electromagnetic induction phenomenon.

It is still another object of the present invention to provide a devicefor detecting a liquid level in a container by putting theelectromagnetic induction phenomenon into practical use.

According to the present invention, there is provided a device formeasuring a displacement of a movable member relative to a fixed member,the device comprising an AC signal source, a first coil mounted to thefixed member and including first and second coil sections which areelectrically connected and coaxially aligned along a common axis so thatthe first coil has a first extreme end constituting an outside end ofthe first coil section, a middle portion defined between respectiveinside ends of the first and second coil sections and a second extremeend constituting an outside end of the second coil section, a secondcoil coaxially disposed in the first coil and mounted to the fixedmember so as to establish a magnetic coupling with first coil, one ofthe first and second coils being connected to the AC signal source toreceive an AC signal, a short circuit ring fixed for movement with themovable member, the short circuit ring being axially disposed about thefirst coil so as to establish a magnetic coupling with the first coiland second coils sand constructed to consume magnetic energy, a floatconnected to the short circuit ring to cause the short circuit ring toremain on top of a fluid, and means connected to the other of the firstand second coils for detecting a change in electromagnetic inductionbetween the first coil and the second coil caused by the short circuitring and for providing an information signal which is representative ofthe change, wherein the winding density of each of the first and secondcoil sections of the first coil is gradually decreased from the outsideend to the inside end thereof, and the winding density of the secondcoil is substantially even throughout its length.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the present invention will becomeapparent from the following description when taken in conjunction withthe accompanying drawings, in which:

FIG. 1 is a sectional view of a prior art liquid level detecting deviceas described hereinabove, with some control circuits connected thereto;

FIG. 2 is a diagramatically illustrated circuit of a liquid leveldetecting device of a first embodiment of the present invention, withseveral control circuits connected thereto;

FIG. 3 is a sectional view of the liquid level detecting device of thefirst embodiment;

FIG. 4 is the circuit of the liquid level detecting device of the firstembodiment with some portions symbolically illustrated;

FIGS. 5a-d are a chart showing various signal forms appearing at givenportions of the circuit of FIG. 2;

FIG. 6 is a graph showing the characteristic of output of the liquidlevel detecting device of the first embodiment;

FIG. 7 is a diagramatically illustrated circuit of a liquid leveldetecting device of a second embodiment;

FIG. 8 is a view similar to FIG. 7, but showing a third embodiment;

FIG. 9 is a view similar to FIG. 3, but showing a fourth embodiment;

FIG. 10 is a view similar to FIG. 7, but showing a fifth embodiment;

FIG. 11 is a graph showing the characteristic of output of the device ofthe fifth embodiment;

FIG. 12 is a view similar to FIG. 7, but showing a sixth embodiment;

FIG. 13 is a graph showing the characteristic of output of the device ofthe sixth embodiment;

FIG. 14 is a view similar to FIG. 2, but showing a seventh embodiment ofthe present invention;

FIG. 15 is a sectional view of a liquid level detecting device of theseventh embodiment;

FIG. 16 is a graph showing the characteristic of output of the device ofthe seventh embodiment;

FIG. 17 is a view similar to FIG. 7, but showing an eighth embodiment ofthe invention;

FIG. 18 is a view similar to FIG. 7, but showing a ninth embodiment;

FIG. 19 is a view similar to FIG. 15, but showing a tenth embodiment ofthe invention;

FIG. 20 is a view similar to FIG. 19, but showing an eleventh embodimentof the invention;

FIG. 21 is a view similar to FIG. 2, but showing a twelfth embodiment ofthe invention;

FIG. 22 is a sectional view of a liquid level detecting device of thetwelfth embodiment;

FIG. 23 is a graph showing the characteristic of output of the device ofthe twelfth embodiment;

FIGS. 24a-i are a chart showing various signal wave forms appearing atgiven portions of the circuit of FIG. 21;

FIG. 25 is a circuit of a liquid level detecting device of a thirteenthembodiment of the invention; and

FIG. 26 is a view similar to FIG. 22, but showing a fourteenthembodiment of the invention;

FIGS. 27 and 28 show modification of the coil arrangements of the firstembodiment shown in FIG. 3; and

FIGS. 29 and 30 show modification of the coil arrangements of the tenthembodiment shown in FIG. 19.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 2 to 6, especially FIG. 2, there is shown aninduction type liquid level detecting device 10A of a first embodimentof the present invention with several control circuits associatedtherewith.

In FIG. 2, denoted by numeral 11 is an AC signal source which suppliesboth a first coil (or exciting coil) 122 and a detection control signalgenerating circuit 13 with an AC signal "a" as depicted by FIG. 5a. Itis to be noted that the frequency of the AC signal "a" (FIG. 5a) is sodetermined that elements (such as, operational amplifier and the like)constituting an after-mentioned amplification circuit 14 can exhibittheir normal performances. More particularly, the frequency can be heldat a low level so long as an undesirable resonance phenomenon due tostray capacitance is avoided. In the disclosed embodiment, the frequencyis set at 1 KHz. By receiving the AC signal "a" (FIG. 5a) from the ACsignal source 11, the circuit 13 issues a pulse signal "c", as shown inFIG. 5c, which is synchronous with the AC signal "a". Denoted by numeral14 is a detecting circuit which comprises an amplifying circuit 141 andan analogue switch 142.

The amplifying circuit 141 amplifies the output signal "b" (FIG. 4b)issued from the induction type liquid level detecting device 10A properwhich will be described hereinafter in detail. The analogue switch 142treats the amplified signal with reference to the detection controlsignal "c" (FIG. 4c) issued from the detection control signal generatingcircuit 13. Denoted by numeral 15 is a filter circuit, 16 is a voltagedetecting circuit and 17 is an indicating circuit. The signal "d" (FIG.4d) treated by the detecting circuit 14 is smoothed by the filtercircuit 15, then detected by the voltage detecting circuit 16 andthereafter treated by the indicating circuit 17 to indicate the amountof the liquid in the container.

The induction type liquid level detecting device 10A will be describedin detail below.

As will be seen from FIG. 3, the device 10A comprises generally thefirst coil 122 (exciting coil) wound on an elongate core 121 (which willbe referred to as a first coil bobbin, hereinafter), a second coil 124wound on a hollow plastic case 123 (which will be referred to as asecond coil bobbin, hereinafter) within which the first coil 122 andthus the first coil bobbin 121 are coaxially housed, and a short circuitring 126 of coil mounted to a float 125. The float 125 is arranged tomove smoothly upward and downward in response to the rise and fall ofthe liquid level in the liquid container. For achieving the smoothmovement of the float 125 relative to the coil-carrying second coilbobbin 123, the float 125 has a guide bore (no numeral) through whichthe second coil bobbin 123 passes with an adequate clearancetherebetween.

As will be understood from FIG. 2, the first coil 122 comprises an upper(or first) coil section 1221 the winding density of which is graduallydecreased from the top portion of the first coil bobbin 121 to themiddle portion of the same, and a lower (or second) coil section 1222the winding density of which is gradually increased from the middleportion of the first coil bobbin 121 to the lower portion of the same.If desired, the winding of the coil sections on the bobbin may be madestepwisely or linearly so long as the winding density inclination isestablished. These two coil sections 221 and 1222 are so arranged as togenerate, upon electric energization, respective magnetic fluxes whichadvance in mutually opposed directions. For achieving this, the lowerend of the first coil section 1221 is connected to the lower end of thesecond coil section 1222, as shown. The upper end of the second coilsection 1222 is earthed. The upper end of the first coil 122 isconnected to the AC signal source 11.

The second coil 124 (measuring coil) is thus magnetically coupled withto the first coil 122. The winding density of the coil 124 is eventhroughout the axial length thereof as is seen from FIG. 2.

As will be seen from FIG. 3, the short circuit ring 126 on the float 125is coaxially arranged with respect to the common axis of the first andsecond coils 122 and 124, establishing a magnetical coupling with thesecoils 122 and 124.

In the following, operation of the induction type liquid level detectingdevice 10A will be described with reference to FIG. 4.

For ease of understanding, let us suppose that the first coil 122serving as an exciting coil comprises a series of small coils L₁, L₂ andL₃ of different number of turns (which series corresponds to the uppercoil section 1221) and another series of small coils L₁ ', L₂ ' and L₃ 'of different number of turns (which series corresponds to the lower coilsection 1222), and let us suppose that the magnetic fluxes produced bythese small coils L₁, L₂, L₃, L₁ ', L₂ ' and L₃ ' are designated by φ₁,φ₂, φ₃, -φ₁, -φ₂ and -φ₃, respectively.

When, as is seen in FIG. 4, the short circuit ring 126 comes to aposition facing the small coil L₂ of the upper coil section 1221 due tofluctuation of the liquid level in the container, the small coils (eachhaving an equal number of turns) constituting the second coil 124 areforced to generate respective induced electromotive forces, which are:##EQU1## wherein: n: the number of turns of each coil.

Because the output of the second coil 124 is represented as a sum of theinduced electromotive forces of these small coils, the output isrepresented by the next equation: ##EQU2##

As will be understood from this equation, when the short circuit ring126 assumes the above-mentioned position, the magnetic energy of thesmall coil L₂ is consumed by the short circuit ring 126 thereby toprevent generation of magnetic flux of the small coil L₂ with a resultthat only the induced electromotive force e₂ ' produced by the smallcoil L₂ ' is outputted from the second coil 2. In this case, as is shownby a solid curved line in FIG. 5b, the electromotive force e₂ ' has thesame phase as the AC signal "a" (FIG. 5a) and has a crest value whichcorresponds to the liquid level then established.

It is to be noted that the signal wave forms illustrated by the dot-dashline and the broken line in FIG. 5b are presented for showing a factthat even when the short circuit ring 126 moves only within the range ofthe upper coil section 1221, the crest value of the inducedelectromotive force outputted from the second coil 124 changes dependingon what magnetic flux of the upper coil section 1221 is shortened by theshort circuit ring 126. It is further to be noted that the signal waveform illustrated by the dot-dot-dash line in FIG. 5b shows the inducedelectromotive force which is outputted from the second coil 124 when theshort circuit ring 126 assumes a position within a range of the lowercoil section 1222. As is seen from this wave form chart, the phase ofthe output (dot-dot-dash line) is reversed to that of the outputproduced when the short circuit ring 126 is located within the range ofthe upper coil section 1221.

As is understood from the foregoing description, the crest value of theoutput signal from the second coil 124 changes in accordance with theposition of the short circuit ring 126, that is, in accordance with theliquid level in the liquid container. Furthermore, the phasic relationof the output signal from the second coil 124 to the AC signal "a" (FIG.5a) changes by 180 degrees depending on whether the short circuit ring126 is within the range of the upper coil section 1221 or the lower coilsection 1222.

The induced electromotive force outputted from the second coil 124 isdetected by the detecting circuit 14 on the basis of the detectioncontrol signal "c" (FIG. 5c) outputted from the detection control signalgenerating circuit 13, and from the detecting circuit 14, there isoutputted a voltage signal "d" with the characteristic as shown by FIG.5d. The voltage signal "d" is applied to the smoothing circuit 15 to besmoothed (see the flat voltage lines designated by +V, +V' and -V inFIG. 5d). Due to the above-mentioned unique arrangement of the firstcoil 122, the DC voltage thus outputted from the smoothing circuit 15shows such the characteristic as shown in FIG. 6 which shows the outputvoltage relative to the position of the liquid level. The value of theDC voltage is read by the voltage detecting circuit 16 and treated bythe indicating circuit 17 to indicate the amount of the liquid in thecontainer.

Although, in the first embodiment 10A, the second coil bobbin 123 isemployed for winding thereon the second coil 124, the present inventionis not limited to such an arrangement. That is, if desired, the secondcoil 124 may be wound directly on the first coil 122 without using thesecond coil bobbin. Furthermore, the positional relation between thefirst coil 122 and the second coil 124 may be reversed. That is, in thisreversed arrangement, the second coil 124 is wound on the first coilbobbin 121 and the first coil 122 is wound on the second coil bobbin 123in which the second coil 124 is installed. Examples of suchmodifications are shown in FIGS. 27 and 28.

Referring to FIG. 7, there is shown a circuit of a liquid leveldetecting device of a second embodiment 10B of the present invention. Inthis second embodiment, unlike the case of the first embodiment, theupper and lower coil sections 1221 and 1222 are connected in parallelwith each other. That is, the inside ends of the upper and lower coilsections 1221 and 1222 are connected and earthed. The parts identical tothose of the first embodiment 10A are denoted by the same numerals inthe drawing, and explanation of them will be omitted.

Referring to FIG. 8, there is shown a circuit of a liquid leveldetecting device 10C of a third embodiment of the invention. As will beseen from this drawing, the exciting coil connected to the AC signalsource 11 is a coil identical to the second coil 124 of the firstembodiment 10A, while the measuring coil is a coil identical to thefirst coil 122 of the first embodiment.

Referring to FIG. 9, there is shown a liquid level detecting device 10Dof a fourth embodiment of the invention. In this embodiment, a metalring 126' constructed of for example aluminium or the like is used inplace of the short circuit ring 126 of coil employed in theabove-mentioned embodiments. The metal ring 126' consumes the magneticenergy on the principle of eddy-current loss. As the short circuit ring126, a ring constructed of magnetic powder-impregnated plastic is alsousable.

Referring to FIG. 10, there is shown a circuit of a liquid leveldetecting device 10E of a fifth embodiment of the invention. In thisembodiment, the lower portion of the device is bent as shown. With this,the output from the smoothing circuit 15 shows the characteristic asshown in the graph of FIG. 11. That is, the left section of the outputcharacteristic curve at which the changing rate of output is quite smallcan be deleted, so that the detecting ability of the device at the timewhen the liquid is small is improved.

Referring to FIG. 12, there is shown a circuit of a liquid leveldetecting device 10F of a sixth embodiment of the invention. In thisembodiment, a rectangular frame-like core 121' is employed. The firstcoil 122 (that is, exciting coil) is wound on one bobbin portion of thecore 121' and the second coil 124 (that is, measuring coil) is wound onthe other bobbin portion to establish a closed magnetic circuit. Withthis arrangement, it is possible to solve a rapid fluctuation of themagnetic field which would occur, due to permeability, at the extremeends of the detecting range. Thus, as is shown by the graph of FIG. 13,the curve of the output characteristic shows at both ends of thedetecting range such characteristic as shown by the solid lines. Thatis, undesirable U-turn phenomenon depicted by the broken lines can besolved. Thus, in the sixth embodiment, wider detecting range is providedas compared with the first embodiment.

Referring to FIGS. 14 and 15, there is shown a liquid level detectingdevice 10G of a seventh embodiment. This device 12G is substantially thesame as that of the first embodiment 10A except for several parts whichwill be described hereinafter. Thus, the substantially same parts asthose of the first embodiment 10A will be denoted by the same numeralsand detailed description of them will be omitted. In the seventhembodiment, two additional coils 1221a and 1222a of increased number ofturns are further employed, one being connected to the upper end of theupper coil section 1221 and the other being connected to the lower endof the lower coil section 1222. As is understood from FIG. 14, thewinding direction of each additional coil 1221a or 1222a is equal to hatof the associated coil section 1221 or 1222. As is seen from FIG. 15,these additional coils 1221a and 1222a are housed in upper and lowerextensions 123a of the second coil bobbin 23, respectively. Preferably,the extensions are formed to have grooves, as shown.

Because of the provision of the additional coils 1221a and 1222a, theoutput from the smoothing circuit 15 has such a characteristic as shownby the solid curve in FIG. 16. For comparison, the output curve of thefirst embodiment 10A is also shown by dot-dot-dash line. As isunderstood from these curves, the detecting range of the seventhembodiment 10G is wider than that of the first embodiment 10A.

Referring to FIG. 17, there is shown an eighth embodiment 10H of theinvention, which is substantially the same as the second embodiment 10B(see FIG. 7) except for the two additional coils 1221a and 1222a. Thatis, in the eighth embodiment 10H, the additional coils 1221a and 1222aare connected to the upper and lower coil sections 1221 and 1222 of thefirst coil 122 in the same manner as the seventh embodiment 10G.

Referring to FIG. 18, there is shown a ninth embodiment 10I of theinvention, which is substantially the same as the third embodiment 10C(see FIG. 8) except for the two additional coils 1221a and 1222a. As isunderstood from the drawing, in the ninth embodiment, the two additionalcoils 1221a and 1222a are connected to the upper and lower coil sections1221 and 1222 of the measuring coil.

Referring to FIG. 19, there is shown a tenth embodiment 10J of theinvention, which is substantially the same as the fourth embodiment 10Dof FIG. 9 except for the two additional coils 1221a and 1222a connectedto the exciting coil 122. The coil arrangements may also be modified, aspreviously discussed, as shown in FIGS. 29 and 30.

Referring to FIG. 20, there is shown an eleventh embodiment 10K of theinvention, which is a modification of the tenth embodiment 10J. That is,in the eleventh embodiment, a cylindrical case 127 is employed forreceiving therein a detector proper which is substantially identical tothe device of the tenth embodiment 10J. The case 127 is formed with asuitable number of holes (no numerals) for, when mounted in a liquidcontainer, providing a fluid communication between the interior of thecase 127 and the exterior of the same. The case 127 is formed with upperand lower stoppers 1271 and 1272 for suppressing extreme movement of thefloat 125.

Referring to FIGS. 21 and 22, there is shown an induction type liquidlevel detecting device 10L of a twelfth embodiment of the invention.

In FIG. 21, denoted by numeral 18 is a first AC signal source and 19 isa second AC signal source. The first AC signal source 18 supplies both amain exciting coil 202 (or first coil) of the induction type liquiddetecting device 10L and a first detection control signal generatingcircuit 21 with an AC signal "a" as shown in FIG. 24a. The second ACsignal source 19 supplies both an auxiliary exciting coil 204 (or thirdcoil) of the device 10L and a second detection control signal generatingcircuit 22 with an AC signal "c" as shown in FIG. 24c. It is to be notedthat the frequencies of the AC signals "a" and "c" are so determinedthat elements (such as operational amplifier or the like) constitutingafter-mentioned amplifying circuits 251 and 261 can exhibit their normalperformances. More particularly, the frequencies can be determined tolow levels so long as an undesirable resonance phenomenon due to straycapacitance is avoided. In the disclosed twelfth embodiment 10L, thefrequency of the AC signal "a" is set at 1 KHz, while the frequency ofthe AC signal "c" is set at 6 KHz. By receiving the AC signal "a", thefirst detection control signal generating circuit 21 issues a pulsesignal "b" (FIG. 24b) of detection control which is synchronous with theAC signal "a" (FIG. 24a). By receiving the AC signal "c" (FIG. 24c), thesecond detection control signal generating circuit 22 issues a pulsesignal "d" (FIG. 24d) of detection control which is synchronous with theAC signal "c". Denoted by numeral 23 is a low-pass filter which receivesan output signal "e" (shown in FIG. 24e) from the device proper 10L anddeletes from the signal "e" a high frequency component corresponding tothe AC signal "c". Thus, a signal "f" having such characteristic asshown in FIG. 24f is outputted from the low-pass filter 23. Denoted bynumeral 24 is a high-pass filter which receives the output signal "e" ofthe device proper 10L and deletes from the signal "e" a low frequencycomponent corresponding to the AC signal "a". Thus, a signal "g" havingsuch a characteristic as shown in FIG. 24g is outputted from thehigh-pass filter 24. Denoted by numeral 25 is a first detection circuitwhich comprises an amplifying circuit 251 for suitably amplifying theoutput signal "f" from the low-pass filter 23 and an analogue switch 252for detecting the amplified output signal "f" in accordance with thedetection control signal "b" issued from the first detection controlsignal generating circuit 21. Denoted by numeral 26 is a seconddetection circuit which comprises an amplifying circuit 261 for suitablyamplifying the output signal "g" from the high-pass filter 24 and ananalogue switch 262 for detecting the amplified output signal "g" inaccordance with the detection control signal "d" issued from the seconddetection control signal generating circuit 22. Denoted by numerals 27and 28 are first and second smoothing circuits which respectively smooththe output signal "h" (see FIG. 24h) from the first detection circuit 25and the output signal "i" (see FIG. 24i) from the second detectioncircuit 26.

Denoted by numerals 29 and 30 are first and second voltage detectingcircuits. The first voltage detecting circuit 29 detects the value ofthe DC voltage signal "h'" (see the line h' in FIG. 24h) outputted fromthe first smoothing circuit 27, while the second voltage detectingcircuit 30 detects the value of the DC voltage signal "i'" (see the linei' in FIG. 24i) outputted from the second smoothing circuit 28. Denotedby numeral 31 is an indication control circuit which permitstransmission of output of the second voltage detecting circuit 30 to anafter-mentioned secondary indicator 322 only when the voltage value ofthe voltage signal "h'" from the first smoothing circuit 27 is lowerthan a predetermined value corresponding to the upper limit (see thepoint L_(o) in the graph of FIG. 23) of liquid level detecting rangewhich is determined when the associated liquid container contains smallamount of liquid therein. That is, only when the voltage value of thevoltage signal "h'" is lower than the predetermined value, the voltagevalue of the voltage signal "i'" from the second smoothing circuit 28 istreated by the secondary indicator 322. Denoted by numeral 32 is adouble function indicator which comprises a primary indicator 321 forrepresenting in bar-graphical fashion the output data (viz., the voltageof the voltage signal "h'") issued from the first voltage detectioncircuit 29 and the above-mentioned secondary indicator 322 forrepresenting in digital fashion the output data (viz., the voltage ofthe voltage signal "i'") issued from the second voltage detectioncircuit 30 through the indication control circuit 31.

In the following, the induction type liquid level detecting device 10Lwill be described in detail.

As will be seen from FIGS. 21 and 22, the device 10L comprises generallythe primary exciting coil (or first coil) 202 wound on a first coilbobbin 201, an auxiliary exciting coil (or third coil) 204 wound on ahollow plastic inner case (or third coil bobbin) 203 (see FIG. 22) inwhich the primary exciting coil 202 is coaxially disposed, a measuringcoil (or second coil) 206 wound on a hollow plastic outer case (secondcoil bobbin) 205 (see FIG. 22) in which the auxiliary exciting coil 204is coaxially disposed, and a short circuit ring 208 of coil fixed to afloat 207 which moves upward and downward in response to the rise andfall of liquid level in the liquid container.

The primary exciting coil 202 has a length sufficiently enough forcovering the liquid level detecting range and comprises upper and lowercoil sections (or first and second coil sections) 2021 and 2022 whichare connected in series and wound on the first coil bobbin 201 extendingvertically. Similar to the afore-mentioned embodiments, the windingdensity of the upper coil section 2021 is gradually decreased from theupper portion of the first coil bobbin 201 to the middle portion of thesame, and the winding density of the lower coil section 2022 isgradually increased from the middle portion of the bobbin 201 to thelower portion of the same. The upper and lower coil sections 2021 and2022 are so arranged as to generate, upon electric energization,respective magnetic fluxes which advance in the mutually opposeddirections. The primary exciting coil 202 is connected to the first ACsignal source 18.

The auxiliary exciting coil 204 has a length of about one third of thatof the primary exciting coil 202 and is wound on a lower portion of thethird coil bobbin 203 (see FIG. 22). The auxiliary exciting coil 204comprises upper and lower coil sections (or third and fourth coilsections) 2041 and 2042 which are connected in series. Similar to theprimary exciting coil 202, the winding density of the upper coil section2041 is gradually decreased from the upper end of the auxiliary excitingcoil 204 to the middle portion of the same, and the winding density ofthe lower coil section 2042 is gradually increased from the middleportion of the auxiliary exciting coil 204 to the lower end of the same.The upper and lower coil sections 2041 and 2042 are so arranged as togenerate, upon electric energization, respective magnetic fluxes whichadvance in the mutually opposed directions. The auxiliary exciting coil204 is connected to the second AC signal source 19.

The measuring coil 206 has a length substantially equal to the primaryexciting coil 202 and is wound on the second coil bobbin 205. Thewinding density of the measuring coil 206 is even throughout the lengththereof, as is understood from FIG. 21. Thus, the measuring coil 206 ismagnetically coupled with both the primary and auxiliary exciting coils202 and 204.

As is seen from FIG. 22, the short circuit ring 208 is coaxiallydisposed on the float 207 which is movable along the length of thesecond coil bobbin 205 in response to the fluctuation of the liquidlevel in the container. Thus, between the short circuit ring 208 andeach of the primary exciting coil 202, the auxiliary exciting coil 204and the measuring coil 206, there is established a magnetic coupling.

In the following, operation of the liquid level detecting device 10Lhaving the above-mentioned arrangement will be described.

First, the description will be commenced with respect to a conditionwherein the liquid level is relatively high, that is, the containercontains a large amount of liquid therein. In this condition, the shortcircuit ring 208 assumes a position to face a part of the upper coilsection 2021 of the primary exciting coil 202, as is shown in FIGS. 21and 22. Thus, for the reason which has been explained in the firstembodiment 10A, only the magnetic flux generated by a part of the lowercoil section 2022 (viz., a counterpart of the part of the upper coilsection 2022 to which the short circuit ring 208 faces) works to makethe measuring coil 206 output the induced electromotive force. It is tobe noted that, in this condition, the auxiliary exciting coil 204 doesnot take part in generation of the induced electromotive force due toabsence of the short circuit ring 208. Accordingly, when the liquidlevel in the liquid container is relatively high, the measuring coil 206outputs an induced electromotive force of the characteristic as depictedby the left-half of the wave-form shown in FIG. 24e. That is, theinduced electromotive force has a phase equal to that of the AC signal"a" issued from the first AC signal source 18 and has a crest valuedetermined in accordance with the distance from the middle portion ofthe primary exciting coil 202 to the liquid level in the liquidcontainer.

Second, the description will be directed to a condition wherein theliquid level is relatively low, that is, the container contains only asmall amount of liquid therein. In this condition, the short ring 208assumes a position to face both the lower coil section 2022 of theprimary exciting coil 202 and the auxiliary exciting coil 204. Thus, amagnetic flux generated by a part of the upper coil section 2021 (viz.,a counterpart of the part of the lower coil section 2022 to which theshort circuit ring 208 faces) of the primary exciting coil 202 andanother magnetic flux generated by either one of upper and lower coilsections 2041 and 2042 work to make the measuring coil 206 output theinduced electromotive force. Accordingly, when the liquid level in theliquid container is relatively low, the measuring coil 206 outputs aninduced electromotive force of the characteristic as depicted by theright-half of the wave-form shown in FIG. 24e. That is, the inducedelectromotive force thus generated is a sum of the induced electromotiveforce which has a phase equal to that of the AC signal "a" issued fromthe AC signal source 18 and has a crest value determined in accordancewith the distance from the middle portion of the primary exciting coil202 to the liquid level in the liquid container and the other inducedelectromotive force which has a phase equal to that of the AC signal "c"issued from the second AC signal source 19 and has a crest valuedetermined in accordance with the distance from the middle portion ofthe auxiliary exciting coil 204 (that is, the middle portion definedbetween the upper and lower coil sections 2041 and 2042) to the liquidlevel in the liquid container.

The induced electromotive force "e" (see FIG. 24e) thus outputted fromthe measuring coil 206 is applied to the low-pass filter 23 and thehigh-pass filter 24. As is seen from the graph of FIG. 24f, uponreceiving the force "e", the low-pass filer 23 outputs a voltage signal"f" the phase of which is synchronous with that of the AC signal "a"from the first AC signal source 18 and the crest value of which isdetermined in accordance with the liquid level in the liquid container.(It is to be noted that the phasic relation of the voltage signal "f"relative to the AC signal "a" changes by 180 degrees depending onwhether the short circuit ring 208 is within the range of the upper coilsection 2021 or the lower coil section 2022. While, the high-pass filter24 outputs, only when the liquid level is low, a voltage signal "g" (seeFIG. 24g) the phase of which is synchronous with that of the AC signal"c" from the second AC signal source 19 and the crest value of which isdetermined in accordance with the liquid level in the liquid container.(It is to be noted that the phasic relation of the voltage signal "f" tothe AC signal "c" changes by 180 degrees depending on whether the shortcircuit ring 208 is within the upper coil section 2041 of the auxiliaryexciting coil 204 or the lower coil section 2042 of the same.

The voltage signal "f" from the low-pass filter 23 is treated by thefirst detection circuit 25 in accordance with the detection controlsignal "b" (see FIG. 24b) issued from the first detection control signalgenerating circuit 21. Thus, a voltage signal "h" having a wave formshown by FIG. 24h is outputted from the first detection circuit 25. Theoutput signal "h" is fed to the first smoothing circuit 27 to besmoothed (see the lines "h'" in FIG. 24h). The DC voltage "h'" thusoutputted from the smoothing circuit 27 has such a characteristic isshown by the curve "h'" of the graph in FIG. 23 due to theabove-mentioned unique winding of the primary exciting coil 202. Thevalue of the DC voltage "h'" is read by the first voltage detectioncircuit 29 and indicated in bar-graphic fashion by the primary indicator321 of the double functioning indicator 32.

While, the voltage signal "g" from the high-pass filter 24 is treated bythe second detection circuit 26 in accordance with the detection controlsignal "d" (see FIG. 24d) issued from the second detection controlsignal generating circuit 22. Thus, a voltage signal "i" having a waveform as shown by FIG. 24i is outputted from the second detection circuit26. The output signal "i" is fed to the second smoothing circuit 28 tobe smoothed (see the line "i'" in FIG. 24i). The DC voltage "i'" thusoutputted from the smoothing circuit 28 has such a characteristic asshown by the curve "i'" of the graph in FIG. 23 due to theabove-mentioned unique winding of the auxiliary exciting coil 204. TheDC voltage "i'" is read by the second voltage detection circuit 30 andindicated in digital fashion by the secondary indicator 322 of thedouble functioning indicator 32.

Referring to FIG. 25, there is shown a circuit of a liquid leveldetecting device 10M of a thirteenth embodiment of the presentinvention. In this embodiment, unlike the case of the twelfth embodiment10L, the upper and lower coil sections 2021 and 2022 of the primaryexciting coil 202 are connected in parallel with each other, and theupper and lower coil sections 2041 and 2042 of the auxiliary excitingcoil 204 are connected in parallel with each other, as shown in thedrawing.

Referring to FIG. 26, there is shown a liquid level detecting device 10Nof a fourteenth embodiment of the present invention. In this embodiment,a metal ring 208' constructed of for example aluminium or the like isused in place of the short circuit ring 208 of coil. The metal ring 208'consumes the magnetic energy on the principle of eddy-current loss. Asthe short ring 208, a ring constructed of magnetic powder-impregnatedplastic is also usable in the invention.

If desired, the following modifications are also possible in theinvention. One of them is a modification in which a suitable condenseris connected in series with the coil of the short circuit ring 208 toprovide a resonance circuit in the same. With this, a current flowing inthe circuit is increased thereby increasing the energy consumption ofthe short circuit ring 208. Accordingly, the output from the measuringcoil 206 is increased.

Although in the above-mentioned embodiments 10A to 10N, the upper andlower coil sections 1221 and 1222 (or 2021 and 2022) of the primaryexciting coil 122 (or 202) and those 2041 and 2042 of the auxiliaryexciting coil 204 are each arranged to be symmetrical with respect tothe middle portion therebetween, the primary and auxiliary excitingcoils 122 (or 202) and 204 may each have an asymmetrical construction solong as the arrangement including afore-mentioned winding densityinclination is employed.

Although, in the foregoing description, the coil bobbins are describedto be constructed of iron or plastics, they may be constructed of othermaterial. If desired, an air-cored arrangement may be used in theinvention.

What is claimed is:
 1. A device for measuring a displacement of amovable member relative to a fixed member, comprising:an AC signalsource; a first coil mounted to said fixed member and including firstand second coil sections which are electrically connected and coaxiallyaligned along a common axis so that the first coil has a first extremeend constituting an outside end of said first coil section, a middleportion defined between respective inside ends of said first and secondcoil sections and a second extreme end constituting an outside end ofsaid second coil section; a second coil coaxially disposed within saidfirst coil and mounted to said fixed member so as to establish amagnetic coupling with said first coil, an end of one of said first andsecond coils being connected to said AC signal source to receive an ACsignal; a short circuit ring fixed for movement with said movablemember, said short circuit ring being disposed upon and located at apredetermined position about said movable member and axially disposedabout said first coil so as to establish a magnetic coupling with saidfirst coil and second coil and constructed to consume magnetic energy; afloat constituting said movable member and being connected to said shortcircuit ring to cause said short circuit ring to remain on top of afluid; and means connected to an end of the other of said first andsecond coils for detecting a change in electromagnetic induction betweensaid first coil and said second coil caused by said short circuit ringand for providing an information signal to a detecting circuit which isrepresentative of said change; wherein the winding density of each ofsaid first and second coil sections of said first coil is graduallydecreased from the outside end to the inside end thereof, and thewinding density of said second coil is substantially even throughout itslength, and wherein the displacement of the float is measured as thefloat moves in response to a parameter.
 2. A device as claimed in claim1, in which said first coil is wound on an outer coil bobbin.
 3. Adevice as claimed in claim 1, in which said first and second coilsections of said first coil are so arranged as to generate, uponelectric energization of said first coil, respective magnetic fluxes inmutually opposed directions.
 4. A device as claimed in claim 1, in whichsaid first and second coil sections of said first coil are so arrangedas to generate, upon electric energization of said second coil,respective induced electromotive forces mutually opposed directions. 5.A device as claimed in claim 1 wherein said detecting means comprises ananalog switch operated in synchronism with said AC signal to passpredetermined portions of said AC signal induced in said second coil,and means for detecting a voltage level of said predetermined portions.6. A device as claimed in claim 5 wherein said detecting means furtherincludes a circuit receiving said AC signal and outputting a controlsignal to said analog switch to operate said analog switch to pass saidpredetermined portions.
 7. A device as claimed in claim 6 wherein saidpredetermined portions are half wave.
 8. A device as claimed in claim 1,in which an end portion of said second coil section which includes saidsecond extreme end and an end portion of said second coil which facessaid end portion of said second coil section are bent with respect tothe common axis.
 9. A device as claimed in claim 1, further comprisingtwo additional coils of increased number of turns, said additional coilsbeing connected to said first and second extreme ends of said firstcoil, respectively.
 10. A device as claimed in claim 1, in which saidshort circuit ring is a closed circuit coil.
 11. A device as claimed inclaim 1, in which said short circuit ring is an annular memberconstructed of electrically conductive material.
 12. A device as claimedin claim 11, in which said annular member is an annular metal plate. 13.A device as claimed in claim 1, in which said first and second coilsections of said first coil are connected in series.
 14. A device asclaimed in claim 1, in which said first and second coil sections of saidfirst coil are connected in parallel.
 15. A device as claimed in claim1, in which said second coil is wound on an inner coil bobbin aboutwhich said outer coil bobbin and said first coil are coaxially disposed.16. A device as claimed in claim 15, in which said inner coil bobbin isof a metal core type.
 17. A device as claimed in claim 15, in which saidouter and inner coil bobbins are constructed of a non-magnetic material.18. A device as claimed in claim 15, further comprising two additionalcoils of increased number of turns, said two additional coils beingrespectively housed in annular recesses formed in axially opposed endportions of said outer coil bobbin.
 19. A device as claimed in claim 18,in which said axially opposed end portions are constructed to serve asstoppers for restricting movement of said movable member.
 20. A deviceas claimed in claim 18, in which said first coil is connected throughsaid additional coils to said AC signal source to be fed with the ACsignal.
 21. A device as claimed in claim 18, in which said second coilis connected through said additional coils to said AC signal source tobe fed with the AC signal.
 22. A device as claimed in claim 18, furthercomprising a cylindrical case which receives therein a unit whichincludes said first and second coils, said two additional coils and theshort circuit ring-mounted movable member, said cylindrical case havinga plurality of holes for, when mounted in a liquid container, providinga fluid communication between the interior of the case and the exteriorof the same.
 23. A device as claimed in claim 22, in which saidcylindrical case is formed with spaced stoppers for restricting movementof said movable member.
 24. A device as claimed in claim 15, furthercomprising a third coil connected to said fixed member and coaxiallydisposed about said first coil to extend along the common axis withinthe range between said middle portion and said second extreme end, saidthird coil including third and fourth coil sections which areelectrically connected and coaxially aligned so that said third coil hasa third extreme end constituting an outside end of said third coilsection, a middle portion defined between respective inside ends of saidthird and fourth coil sections and a fourth extreme end constituting anoutside end of said fourth coil section, wherein said third and fourthcoil sections of the third coil are so arranged as to generate, uponelectric energization, respective magnetic fluxes which advance in themutually opposed directions, and wherein the winding density of each ofsaid third and fourth coil sections is gradually decreased from theoutside end to the inside end thereof.
 25. A device as claimed in claim24, in which said first coil is connected with said AC signal source tobe fed with an AC signal and in which said third coil is connected withanother AC signal source to bed fed with an AC signal.
 26. A device asclaimed in claim 24, in which said first and second coil sections ofsaid first coil are connected in series.
 27. A device as claimed inclaim 26, in which said third and fourth coil sections of said thirdcoil are connected in series.
 28. A device as claimed in claim 27, inwhich said third and fourth coil sections of said third coil areconnected in parallel.
 29. A device as claimed in claim 1, in which saidsecond coil is wound on an inner coil bobbin.
 30. A device as claimed inclaim 29, in which said first coil is directly wound on said secondcoil.
 31. A device as claimed in claim 30, in which the inner coilbobbin is of a metal core type.
 32. A device as claimed in claim 30, inwhich said inner coil bobbin is constructed of a non-magnetic material.33. A device as claimed in claim 30, in which said two additional coilsare arranged at axially opposed end portions of said inner coil bobbin.34. A device as claimed in claim 33, in which said first coil isconnected through said additional coils to said AC signal source to befed with the AC signal.
 35. A device as claimed in claim 33, in whichsaid second coil is connected through said additional coils to said ACsignal source to be fed with the AC signal.
 36. A device as claimed inclaim 33, further comprising a cylindrical case which receives therein aunit which includes said first and second coils, said two additionalcoils and the short circuit ring-mounted movable member, saidcylindrical case having a plurality of holes for, when mounted in aliquid container, providing a fluid communication between the interiorof the case and the exterior of the same.
 37. A device as claimed inclaim 36, in which said cylindrical case is formed with spaced stoppersfor restricting movement of said movable member.
 38. A device as claimedin claim 30, further comprising a third coil connected to said fixedmember and coaxially disposed about said first coil to extend along thecommon axis within the range, between said middle portion and saidsecond extreme end, said third coil including third and fourth coilsections which are electrically connected and coaxially aligned so thatsaid third coil has a third extreme end constituting an outside end ofsaid third coil section, a middle portion defined between respectiveinside ends of said third and fourth coil sections and a fourth extremeend constituting an outside end of said fourth coil section, whereinsaid third and fourth coil sections of the third coil are so arranged asto generate, upon electric energization, respective magnetic fluxeswhich advance in the mutually opposed directions, and wherein thewinding density of each of the third and fourth coil sections isgradually decreased from the outside end to the inside end thereof. 39.A device as claimed in claim 38, in which said first coil is connectedwith said AC signal source to be fed with an AC signal and in which saidthird coil is connected with another AC signal source to be fed with anAC signal.
 40. A device as claimed in claim 1, further comprising athird coil connected to said fixed member and extending along the commonaxis within the range between said middle portion and said secondextreme end, said third coil including third and fourth coil sectionswhich are electrically connected and coaxially aligned so that saidthird coil has a third extreme end constituting an outside end of saidthird coil section, a middle portion defined between respective insideends of said third and fourth coil sections and a fourth extreme endconstituting an outside end of said fourth coil section, wherein saidthird and fourth coil sections of the third coil are so arranged as togenerate, upon electric energization, respective magnetic fluxes whichadvance in the mutually opposed directions, and wherein the windingdensity of each of said third and fourth coil sections is graduallydecreased from the outside end to the inside end thereof.
 41. A deviceas claimed in claim 40, in which said first and second coil sections ofsaid first coil are connected in parallel.
 42. A device as claimed inclaim 41, in which said third and fourth coil sections of said thirdcoil are connected in series.
 43. A device as claimed in claim 41, inwhich said third and fourth coil sections of said third coil areconnected in parallel.